Oppo — file not delete

Address:

Ahmedabad, Gujarat, 91

Sir, kindly give me a solution why the files cannot deleted from my oppo a37 device. Well it is work while i deleted some other files from device storage. But it doesn't work while file is stored in sd card. It is creatino so many problems as my mobile memory is going to full. And it is starting slow down the system day by day. So, please sir kindly resolve the problem as soon as you can. Thank you.